In motor vehicles, numerous decorative parts are employed which optionally also assume functional jobs in the interior of the vehicle, but which must in particular be of a high-quality optical appearance. This in particular applies to steering wheels and comparable components that comprise an external visible side and at least two support parts connected along a connecting joint. The support parts are provided with a decorative layer facing the visible side. The decorative layer itself is in turn provided with a transparent cover layer on the visible side. The decorative layer can be formed, for example, of precious wood veneer, woven, sheet metal, or the like whose optical appearance shows to advantage through the transparent cover layer. The transparent cover layer here also functions as a protection of the decorative layer and the provision of the desired haptic properties.
Taking into consideration the high demands of the optical and haptic qualities and resistance to ageing, the production expenditure for such decorative parts is high. The two support parts, including the decorative layers applied on it, must be connected or glued to each other at the connecting joint so neatly that a connecting seam is formed that is optically and haptically barely perceptible. This requires one or several complex working steps. In addition, the transparent cover layer, for which a thick-film clear varnish is often used, must be applied. In production, the latter requires a cost-intensive manual operation, in particular during reworking. Therefore, efforts are being made to produce the transparent cover layer by so-called “in mold coating” with an out-of-tool surface. In the process, the blank of the decorative part is placed into an injection mold, and the transparent plastic material is injected for forming the cover layer. In such production methods, two different plastics are employed.
In a first variant, a thermoplastic is fused in an extruder and injected into the injection mold at high pressure. Since the thermoplastic is formed of long-chain polymers and is highly viscous even in the form of a melt, high injection pressures and temperatures are required. The blank inserted in the mold must be able to withstand these pressures and temperatures which is why such a method cannot be employed for any decorative part. In particular, those decorative parts having a cross-section with a circumferentially enclosed visible side cannot be coated, as is the case, for example, with the rim of a steering wheel. The rim of a steering wheel typically has a supporting metal core around which shock-absorbing rigid foam is foamed. The rigid foam, including its decorative layer applied on the outer side, would yield under the influence of the high injection pressure, so that no dimensional stability and deformation resistance can be ensured.
As an alternative, so-called reaction plastics are employed wherein at least two separate plastic components are mixed in a mixing head arranged directly at the outer side of the injection mold, whereby then a chemical reaction is started that leads to a polymer reaction (polyaddition, polymerization). The mixed components are injected into the injection mold. As long as the polymer reaction is not completed, the viscosity of the mixture is low compared to a plastic melt, so that lower injection pressures can be applied. It is thus possible to also coat more sensitive blanks of decorative parts by injection.
Here, however, numerous disadvantages also arise: The curing or polyaddition process takes some time within which the blank coated by injection must remain in the injection mold thereby limiting cycle times in production. To nevertheless obtain a short cycle time, reaction plastics with a very short pot life of about four to seven seconds are prepared. The pot life is thus shorter than the cycle time during which the workpiece or the decorative part remains in the die. While the finished decorative part can then be removed from the mold, partly or completely cured plastic material is also present at the outlet of the mixing head and can lead to plug formation or clogging of the injection channel. At least, however, there is a risk in that individual cured particles get into the mold cavity during the subsequent injection operation and lead to optical impurities there and thus to a deterioration of product quality.
The high affinity of the reaction plastics to metal is also problematic. The injected plastic material tends to adhere to the surface of the mold cavity. Ejection of the finished component is difficult which is why parting agents must be used. In this case, however, despite any efforts, no real out-of-tool surface is formed because the additional use of a parting agent is required. One has also tried to provide the reaction plastic with a separating additive. The additive is initially uniformly distributed in the plastic material and develops its whole effect only after a certain time that it needs to deposit at the surface. With the aforementioned short pot lives, however, the available period is much too short to achieve a sufficient development of the separating effect of the additive.
A further problem not yet solved in prior art is that with decorative parts with at least two support parts, the latter must be precisely connected to each other in a complicated operation before the decorative layer is injected around them, so that the injection result is also satisfactory in the region of the seam. The at least two support parts must be joined to each other so neatly in a separate preparing working step, for example by ultrasonic welding or by gluing, that the decorative layers applied onto it also adjoin practically without any seam, while no detachments, fractures, and in particular impurities of the visible side occur, for example by the employed adhesive. Therefore, despite the high efforts, a cost-boosting amount of rejects is inevitable.